The present invention relates, in general, to metal nanocrystals and methods of fabricating the same, and more particularly to semiconductor memory devices incorporating metal nanocrystals to provide a multibit-per-cell storage capability.
Conventional DRAM or Flash memories have been widely employed for many years in a variety of applications. DRAMs allow fast write/erase, but suffer from high power consumption incurred by the constant refresh operations due to their short retention time of less than a few seconds (see B. El-Kareh et al, “The Evolution of DRAM Cell Technology”, Solid State Technology, Vol. 40, pp 89, May 1997). It is also very difficult to fabricate high-density DRAMs, because a large capacitor is necessary for every cell for charge retention and sufficient perturbation of the bit line to trigger sense amplification during reading. Flash memories, which offer longer than 10 years of retention time, have the drawbacks of high operation voltage and slow write/erase because of their relatively thick tunnel oxide. Known nanocrystal memories and MNOS (SONOS) memory devices employing discrete charge traps as storage elements have exhibited great potential in device performance, power consumption, and technology scalability, thus recently attracting much research attention as promising candidates to replace the conventional DRAM or Flash memories. However, such devices have not solved all of the problems inherent in such devices.